Phosphate coatings have been widely used in coating base steel sheets for use in the manufacturing of automobiles, home appliances, and the like. In the case of such plated steel sheets used in the manufacturing of automobiles, home appliances, and the like, since coating adhesion force is relatively low in the plated steel itself, a phosphate treatment is generally carried out to improve coating adhesion.
A mechanism of the formation of phosphate coating is that a plating layer melts on a portion of a coating object having positive polarity and an insoluble phosphate crystal is precipitated on a portion of a coating object having negative polarity. However, in a case in which a plating layer is damaged due to an excessive etching reaction in a phosphate solution treating process, the base steel sheet may be exposed, such that a pitting phenomenon may occur therein.
Patent Document 1 discloses that since chloride ions contained in a phosphate solution lead to the occurrence of such a pitting phenomenon, the content of chloride ions in a phosphate solution should be decreased in order to suppress the occurrence of pitting.
As such, the chloride ions contained in a phosphate solution further promote an etching reaction at the time of the formation of a phosphate film to thus increase the occurrence of pitting. Thus, in order to suppress the occurrence of pitting, a chloride ion concentration within the phosphate treatment solution needs to be significantly lowered, but since chloride ions are commonly present in city water supplies or a raw materials used when a phosphate solution is produced in the form of impurities, it may be difficult to appropriately adjust a chloride ion concentration.
Meanwhile, relatively low corrosion resistance of a phosphate film itself may cause a problem such as the corrosion of products while phosphate-treated products are being stored or transported, and a porous phosphate film may have inferior corrosion resistance. Therefore, technology for improving the corrosion resistance of a phosphate film through a separate sealing process using hexavalent chromium has been developed. However, as the use of hexavalent chromium is closely regulated due to environmental concerns, improvements in the corrosion resistance of a phosphate film through chromium sealing may not be obtained.
Accordingly, the development of a phosphate solution able to improve corrosion resistance while effectively suppressing pitting occurring due to chloride ions contained in a solution, without using hexavalent chromium, has been urgently demanded.